*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
.clearfix:before,
.clearfix:after{
    display: block;
    content: "";
    clear: both;
}
.main{
    width: 1200px;
    margin: 20px auto 0 auto;
}
.col{
    background: lightblue;
    width: 33.333333%;
    float: left;
    height: 100px;
}
@media (max-width: 600px) {
    .main{
        width: 90%;
    }
    .col{
        width: 100%;
        margin: 5px 0;
    }
}